Because of the media.....
After showing the 20 super slow motion replays, snicko, hotspot, (all designed to prove a wrong desision) etc you can see sometimes maybe a desision was wrong - then the commentators/pundits go on about what a 'shocker' it was for the rest of the game.....
The same is happening in every sport, not just cricket. It just makes me laugh when the commentator isnt willing to give an opinion on a decision until they have seen the replays.
Anyway, back to the point, because of this people now question the desisions because thats whats done on TV!